tried signing up for an account it said the bonus code was expired.....: stars50.

I double checked from PokerStars that the 100% up to $50 sign up bonus code is: Stars50

Please try typing it again.

As HeyKidMove said, traditional rakeback to PokerStars is not available, but they do have an internal VIP program that awards points to you that you can change for cash at higher VIP levels or use to buy tournament tickets. The return per rake can get close to in effectiveness to rakeback deals.